Electromagnetically driven actuators can be operated with low tolerance in the so-called full stroke operating mode. This means that an armature of the actuator is moved to and fro between a starting position and an end position. The starting position and end position are each typically defined here by a mechanical stop of the armature on a housing of the actuator. With respect to an example of an injection valve for injecting fuel, this operating mode means that a valve needle of the injection valve is respectively moved up to a maximum deflection. The injected quantity of fuel is then varied by suitably adapting the duration of the injection process.
However, in order to reduce emissions of pollutants and/or the consumption of fuel by motor vehicles it is necessary in modern injection systems to control the operation of injection valves as precisely as possible, even in the case of small injection quantities. This means that what is referred to as the ballistic operating mode of an injection valve is also controlled. The ballistic operating mode of an injection valve is understood in this context to be partial deflection of the armature or of the valve needle in a trajectory which is predefined by electrical and/or structural parameters and is free, i.e. parabolic, after the ending of the electromagnetic application of force to the armature, without reaching the full stop.
In contrast to the full stroke operating mode, the ballistic operating mode of an injection valve is subject to tolerances to a significantly greater degree, since here, both electrical and mechanical tolerances influence the opening profile to a substantially greater degree than is the case in the full-stroke operating mode. For the ballistic operating mode of an injection valve, generally of an electromagnetically driven armature of an actuator comprising a coil, the following tolerances may occur here, individually or in combination with one another:
a) Opening tolerance: the time at which the armature moves away from its starting position after a defined electrical actuation pulse has been applied to the coil depends on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof (for example temperature).
b) Closing tolerance: the time at which the armature returns again to its starting position after a partial deflection depends on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof.
c) Stroke tolerance: In the case of a partial deflection of the armature, the maximum stroke reached depends likewise on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof. The stroke tolerance brings about an individual change in the parabolic trajectory of the armature with the possibility of the corresponding deflection curve being undesirably flattened or excessively increased.
DE 10 2006 035 225 A1 discloses an electromagnetic actuating device which has a coil. The actual movement of the actuating device can be analyzed by evaluating induced voltage signals which are caused by external mechanical influences.
DE 198 34 405 A1 discloses a method for estimating a needle stroke of a solenoid valve. During the movement of the valve needle in relation to a coil of the solenoid valve, the voltages induced in the coil are sensed and placed in relationship with the stroke of the valve needle by means of a computational model. The derivative over time dU/dt of the coil voltage can be used to determine the contact time since this signal has large jumps at the reversal point of the needle movement or armature movement.
DE 38 43 138 A1 discloses a method for controlling and sensing the movement of an armature of an electromagnetic switching element. When the switching element is switched off, a magnetic field in the exciter winding thereof is induced, said magnetic field being changed by the armature movement. The changes over time in the voltage applied to the exciter winding, which are due to said armature movement, can be used to sense the end of the armature movement.